Closed mold processing is a plastic processing technique which has heretofore been utilized with consistent reproducibility to produce complex, high-strength shapes having a smooth finish on both sides. A typical closed mold apparatus utilizes a two-piece mold consisting of a female die or cavity and a male die or core. A composite of resin, reinforcement, fillers and additives is introduced between the matched dies which are heated and closed under pressure to cure the composite into a solid reinforced plastic part. There are four basic closed mold processes including compression molding, injection molding, stamping, and resin transfer molding.
Resin transfer molding, also commonly referred to as RTM, is a process that has heretofore been typically used in low to medium volume applications. However, recent improvements in resin chemistry are making the process cost-effective for higher volume applications. The reinforcement used in resin transfer molding typically is a mat reinforcement. The tooling used therein can be heated or non-heated, machined and matched metal molds, or warm matched plastics. When molding, the reinforcement typically is placed on the lower mold, the mold halves clamped together, and resin pumped into the molds under pressure. A compressible seal is built into the mold around the part periphery, with provisions for allowing air to escape while containing the resin.
Most known prior art resin transfer molding apparatus typically use a small metering pump, having a resin cylinder for one-part systems and separate resin and catalyst cylinders for two-part resin systems, to fill the mold through a hand-held static mixer that is placed adjacent to or is temporarily fastened to an injection port on the mold. The resin is pumped in by repeated stroking of the metering cylinders until a predetermined volume has been delivered into the mold. The static mixer then is withdrawn and flushed out. However, the above-described prior art method and apparatus has several disadvantages when compared to the apparatus and process of the present invention. First, the metering pump of the present invention is a single-shot metering pump having a closed-loop hydraulic control, which enables the injection rate and pressure of the resin to be profiled or varied during the injection cycle. This results in a more consistent fill of the mold over that obtained in prior art apparatus and methods, and eliminates part waviness caused by pressure rise and fall as a result of the repeated stroking of prior art devices. Moreover, under proper conditions, attachment of the injector assembly to the mold and placement of a plurality of valves in the apparatus enables unused material to be recirculated back to the resin supply tank, and further allows the entire apparatus to be automatically controlled by a computer resulting in an entirely automated system. If desired, a conventional industrial computer or central processing unit which can be used to automatically control the mold clamp or press, can be interfaced with the computer of the apparatus of the invention to further automate the manufacturing process for parts molded by the resin transfer molding method. In addition, the attached injector assembly and valves permit switching between different resins by automatic resin or solvent flushing of the lines of the apparatus, which eliminates draining and refilling of the entire apparatus, which is necessary with known prior art equipment.
U.S. Pat. No. 4,386,899 to Cook relates to a material injection assembly of the type for injecting a resin material into a mold cavity including a body member defining a nozzle for injecting material into a mold and including at least one central channel for conveying a first material through the nozzle. The assembly is characterized by an internal purging mechanism for closing the nozzle to the mold while allowing the flow of purging material between the channel supplying resin material to the control channel and a debris channel. The purging mechanism further allows flow of purging material between a separate purging material channel and the debris channel to perform a post-purge function.
U.S. Pat. No. 4,611,983 to Bielfeldt relates to an injection press for use in a transfer molding system for fiber reinforced thermosetting resins, comprising an injection piston, an injection cylinder in which the injection piston moves, a feed bore which traverses the injection cylinder, a sprue orifice which leads into the mold, a telescoping sleeve which serves to seal off the feed bore when the piston travels toward the sprue orifice, wherein the injection piston seals the transfer bore at the end of the stroke near the sprue orifice, and wherein said feed bore is open when the piston is in a position a stroke length away from the sprue orifice.